Btrfs: avoid taking the chunk_mutex in do_chunk_alloc

Everytime we try to allocate disk space we try and see if we can pre-emptively
allocate a chunk, but in the common case we don't allocate anything, so there is
no sense in taking the chunk_mutex at all.  So instead if we are allocating a
chunk, mark it in the space_info so we don't get two people trying to allocate
at the same time.  Thanks,

Signed-off-by: Josef Bacik <josef@redhat.com>
Reviewed-by: Liu Bo <liubo2009@cn.fujitsu.com>

2 files changed, 28 insertions, 6 deletions

diff --git a/fs/btrfs/ctree.h b/fs/btrfs/ctree.h
index 0d00a07b5b29..2e61fe1b6b8c 100644
--- a/fs/btrfs/ctree.h
+++ b/fs/btrfs/ctree.h
@@ -740,8 +740,10 @@ struct btrfs_space_info {
          */
         unsigned long reservation_progress;
 
-        int full;               /* indicates that we cannot allocate any more
+        int full:1;             /* indicates that we cannot allocate any more
                                    chunks for this space */
+        int chunk_alloc:1;      /* set if we are allocating a chunk */
+
         int force_alloc;        /* set if we need to force a chunk alloc for
                                    this space */
 
diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
index 26479484180d..31f33ba56fe8 100644
--- a/fs/btrfs/extent-tree.c
+++ b/fs/btrfs/extent-tree.c
@@ -3039,6 +3039,7 @@ static int update_space_info(struct btrfs_fs_info *info, u64 flags,
         found->bytes_may_use = 0;
         found->full = 0;
         found->force_alloc = CHUNK_ALLOC_NO_FORCE;
+        found->chunk_alloc = 0;
         *space_info = found;
         list_add_rcu(&found->list, &info->space_info);
         atomic_set(&found->caching_threads, 0);
@@ -3318,10 +3319,9 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans,
 {
         struct btrfs_space_info *space_info;
         struct btrfs_fs_info *fs_info = extent_root->fs_info;
+        int wait_for_alloc = 0;
         int ret = 0;
 
-        mutex_lock(&fs_info->chunk_mutex);
-
         flags = btrfs_reduce_alloc_profile(extent_root, flags);
 
         space_info = __find_space_info(extent_root->fs_info, flags);
@@ -3332,21 +3332,40 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans,
         }
         BUG_ON(!space_info);
 
+again:
         spin_lock(&space_info->lock);
         if (space_info->force_alloc)
                 force = space_info->force_alloc;
         if (space_info->full) {
                 spin_unlock(&space_info->lock);
-                goto out;
+                return 0;
         }
 
         if (!should_alloc_chunk(extent_root, space_info, alloc_bytes, force)) {
                 spin_unlock(&space_info->lock);
-                goto out;
+                return 0;
+        } else if (space_info->chunk_alloc) {
+                wait_for_alloc = 1;
+        } else {
+                space_info->chunk_alloc = 1;
         }
 
         spin_unlock(&space_info->lock);
 
+        mutex_lock(&fs_info->chunk_mutex);
+
+        /*
+         * The chunk_mutex is held throughout the entirety of a chunk
+         * allocation, so once we've acquired the chunk_mutex we know that the
+         * other guy is done and we need to recheck and see if we should
+         * allocate.
+         */
+        if (wait_for_alloc) {
+                mutex_unlock(&fs_info->chunk_mutex);
+                wait_for_alloc = 0;
+                goto again;
+        }
+
         /*
          * If we have mixed data/metadata chunks we want to make sure we keep
          * allocating mixed chunks instead of individual chunks.
@@ -3372,9 +3391,10 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans,
                 space_info->full = 1;
         else
                 ret = 1;
+
         space_info->force_alloc = CHUNK_ALLOC_NO_FORCE;
+        space_info->chunk_alloc = 0;
         spin_unlock(&space_info->lock);
-out:
         mutex_unlock(&extent_root->fs_info->chunk_mutex);
         return ret;
 }